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We proposed a novel photosensitive hydrogel based holographic sensor for the detection of hydrogel swelling and hence the chemical concentrations, which formed by thermally polymerizing 2-hydroxyethyl methacrylate. Holographic volume grating recorded in the hydrogel as an optical sensing platform was employed in response to the hydrogel deformation induced by target molecules. Co-monomer methacrylic acid with carboxyl groups functionalized the polymer matrix for improving the selectivity. Peak wavelength shift of the diffraction spectrum was a significant parameter in sensing the pH value of the buffer. The extracted time constant of the wavelength shift curve evaluated the sensitivity of the novel holographic sensor. There were good exponential relations between peak wavelength and pH value with various thicknesses. The novel sensor could be considered as an effective platform for sensing the analytes concentration of solution. The reversible measurement experimentally demonstrated the repeatability of the holographic sensor. The novel photosensitive hydrogel and its sensor provided a probability for sensing the performance of solution and accelerating the development of water resistant holographic sensor based on polymer hydrogel.

Optical sensor based on pH-sensitive hydrogel has important practical applications in medical diagnosis and bio-sensor areas. This report details the experimental and theoretical results from a novel photosensitive polymer hydrogel holographic sensor, which formed by thermal polymerization of 2-hydroxyethyl methacrylate, for the detection of pH in buffer. Volume grating recorded in the polymer hydrogel was employed in response to the performance of solution. Methacrylic acid with carboxyl groups was selected as the primary co-monomer to functionalize the matrix. Peak diffraction spectrum of holographic grating determined as a primary sensing parameter was characterized to reflect the change in pH. The extracted linear relation between peak wavelength and pH value provided a probability for the practical application of holographic sensor. To explore the sensing mechanism deeply, a theoretical model was used to describe the relevant holographic processes, including grating formation, dark diffusional enhancement, and final fringe swelling. Numerical result further showed all of the dynamic processes and internal sensing physical mechanism. These experimental and numerical results provided a significant foundation for the development of novel holographic sensor based on polymer hydrogel and improvement of its practical applicability.

The LiNbO 3 crystal co-doped with ZrO 4 and Fe 2 O 3 has been grown with [Li]/[Nb]=0.85 and 1.20, respectively, by the Czochralski method in air atmosphere. The incident exposure energy flux threshold for the light-induced scattering was characterized to investigate the scattering properties of the crystals. Applying the results of the incident exposure energy flux threshold effect, the photorefractive properties at different laser wavelengths (473 nm and 532 nm) were also measured by using the typical two-wave coupling experiments. The results show that Zr:Fe:LiNbO 3 crystal has a larger refractive index change, higher recording sensitivity and larger two-wave coupling gain coefficient at 473 nm wavelength than those obtained at 532 nm wavelength under the same experimental conditions. Moreover, the photorefractive properties decrease with the increasing [Li]/[Nb] ratios. The material of Zr:Fe:LiNbO 3 crystal is a promising candidate for blue photorefractive holographic recording.

A series of Hf:Fe:LiNbO3 crystals were grown by the Czochralski technique with various doping concentrations of HfO2. Their defect structures were analyzed by the UV–visible absorption spectra and infrared absorption spectra. The optical damage resistance of Hf:Fe:LiNbO3 crystals was measured by the photo-induced birefringence change and the transmitted light spot distortion method. The results show that the optical damage resistance ability of Hf:Fe:LiNbO3 crystals enhances remarkably with the HfO2 concentration increasing when the HfO2 concentration is lower than its threshold concentration (4 mol%). However, when the HfO2 concentration exceeds its threshold concentration, the optical damage resistance ability of the crystals returns to decrease. This unusual behavior is explained by using the photovoltaic field produced in the crystals.